1. Field of the Invention
The present invention relates to systems and methods for measuring exhaust gas flow rate of a vehicle.
2. Background Art
An accurate determination of the exhaust gas volumetric flow rate of a vehicle is useful for hardware, software, and calibration development in addition to testing compliance with various environmental regulations. While measurement of exhaust gas flows under actual operating conditions is preferable, it is often impractical with currently available devices. Instead, a vehicle or engine is operated under controlled conditions to simulate actual driving or operating conditions. Vehicle exhaust gases exhibit a wide range of flow rates and temperatures corresponding to varying operating modes and ambient conditions, which presents challenges to accurate flow rate measurement. Although a number of devices have been developed to measure exhaust gas flow rates, all have various disadvantages.
Hot wire anemometer-type devices have been used to provide exhaust gas flow rate measurement, but experience measurement errors associated with pulsating or reversing flows that may occur during idling of gasoline engines, for example. In diesel engine applications, these devices may experience soot accumulation contributing to measurement errors and durability concerns. Conventional differential pressure devices and laminar flow devices place one or more obstructions in the exhaust stream to generate a pressure drop and determine flow based on the differential pressure between upstream and downstream pressure transducers. However, these devices produce an undesirable increase in back pressure, which may adversely affect engine operation and result in measurements that do not reflect actual operating conditions. In addition, laminar flow devices are typically too heavy for portable use on a vehicle without additional reinforcement, contribute undesirable thermal capacity to the system, and do not have the desired operating range for use as a portable on-board measuring device. Formation of condensation on the flow measuring devices, especially during cold starts, may also result in measurement errors.